Since 1950, rapid improvements in the field of magnetic recording have been achieved. This highly developed method of magnetic recording involves the use of a particulate magnetic construction, which utilizes magnetization directions predominantly in the plane of the medium. Improvements that have been made include improved head design, the use of media with higher coercivities and smoother surfaces.
One of the main aims of research into magnetic recording systems has been to increase information densities or storage capacity. The degree of storage that is possible is related to the packing density of the magnetic materials in the particulate constructions. A current system of magnetic recording employs small discrete magnetic particles, dispersed in an organic binder. However, due to rapid improvements in this technology the amount of information that can be recorded is now reaching its theoretical limit. Hence other methods of increasing the magnetic density are being investigated.
One type of recording media of interest is perpendicular recording. This particular technology is possible due to the discovery that certain continuous magnetic films such as CoCr, have a perpendicularly orientated crystalline anistropy, which is large enough to overcome demagnetization. High density packing can be achieved due to the CoCr alloy microstructure, which provides magnetically isolated small grains. The grain diameter is approximately 50 nm, employing a maximum density of approximately 500 kilo flux changes per inch (200 kilo flux changes per centimetre). Due to this property of the alloy, much higher densities can be achieved compared to conventional particulate systems, with an additional advantage of a much smoother surface, and hence improved head to media contact. Such media are referred to herein as metallic thin film magnetic media.
The magnetic thin film magnetic media can be formed by sputtering, evaporation, etc., from many different suitable alloys, such as CoCr, CoNi, CoCrNi etc. However, although these metallic films are very promising as high density recording media, they are susceptible to surface abrasion, and additionally cause excessive head wear. Transducer heads contacting the metallic thin film will have a tendency to erode or otherwise damage the metallic thin film. Even slight erosion will result in considerable loss of data when high bit density recording is employed. Applications likely to cause erosion and severe wear of metallic thin film media are on the increase, with video tape and electronic cameras being two examples. It is therefore essential to protect the media with some form of protective coating. However, due to losses that occur in the recording and reading processes when the recording or reading head is separated from the magnetic layer, it is desirable what the total thickness of any overlying layers on the recording layer should be less than 20 nm.
Various attempts have been made to alleviate the problem of alloy wear.
Lubricating systems that have been used as protecting layers for thin film magnetic media include metal layers, using various soft and hard non-magnetic metals, organic layers and mixtures of both. These layers have been applied using various techniques. Metallic layers are usually sputter or vapor deposited, whereas the organic materials are generally coated from solvent, using techniques such as dip coating or spin coating.
Different types of single layer metallic protective coatings are disclosed in Japanese Patent Application Nos. JP62-112211, JP62-112210, JP61-210516, JP61-104317 and JP61-123015.
Multi-layer metallic protective systems are disclosed in Japanese Patent Application Nos. JP60-229223, JP61-120341 and JP61-211826. However, although these materials offer improved durability and system endurance for the thin magnetic media, compared to the magnetic surface with no protection, these hard protective layers may still inflict severe head abrasion. Another disadvantage in using such a system is the cost of producing such metallic protecting layers. Therefore, a great deal of research has been directed towards totally or partially solvent coated lubricant systems.
Organic lubricants e.g., fatty acids and esters, such as carnauba wax have been used as protective lubricating layers both for thin film magnetic media and media comprising particulate material dispersed in a binder. These materials were considered to be satisfactory at ordinary ambient conditions, but were not reliable at high temperatures and relative humidities, e.g. 40.degree. C. and 80% RH. Also, it was discovered that minor variations during the use of these materials, such as in the coating concentration, can destroy their utility. Minor improvements were obtained with silicones, as disclosed in U.S. Pat. No. 2,654,681. These were usable at higher temperatures and humidities, but still were not entirely satisfactory.
U.S. Pat. No. 3,490,946 discloses the use of fluorocarbon compounds as lubricants for flexible magnetic recording tapes. U.S. Pat. Nos. 3,778,308, 4,267,238 and 4,268,556 disclose the use of perfluoro-polyethers (PPE) as lubricant protective layers for magnetic media. Improved wear resistance and corrosion protection were obtained, although some problems were encountered in firmly bonding the top coat lubricant to the magnetic recording layer.
The prior art generally teaches that the topcoat lubricant should be firmly anchored to the magnetic recording layer so that it cannot be easily removed. Removal would obviously reduce the lubricating powers of the surface coating, and/or cause clogging of the recording/playback heads, thus leading to a deterioration in the performance. The prior art teaches that an improvement in lubricant adhesion to the magnetic surface can be achieved by introducing a priming layer between the magnetic layer and the fluorinated topcoat.
The utility of such a primer layer is disclosed in U.S. Pat. No. 4,404,247, where its use improves the binding characteristics of the lubricant, and hence improves the endurance of the magnetic media. The aforementioned patent discloses a dual layer system consisting of an aromatic or heterocyclic polymerisable monomer and a vinyl aromatic polymer, sandwiched between the magnetic layer and the fluorinated topcoat.
Other solvent coated primer materials have been based on silane containing compounds. These include alkoxy silanes which were used with and without phosphate accelerating agent as disclosed in Japanese Patent Application Nos. JP60-029934 and JP60-038729, respectively. These systems were then topcoated with ethyl stearate in JP60-029934 and PPE in JP60-038729. U.S. Pat. No. 4,529,569 discloses the use of a primer layer of an amino functional alkoxy silane and a top coat of a functional PPE in which the functional group is a terminal carboxyl or sulfonic acid group.